For the purpose of the descriptions herein the invention is described in reference to a rock drill although it is apparent that it may be used as well with other types of drilling machines.
It is common in rock drills or other drilling machines to provide a centralizer or collaring means at the working extremity of the drill steel adjacent to the point at which the drill bit and drill steel enter the material being drilled to maintain the drill in centered relation to the hole being drilled. In large drills, particularly those designed and used for drilling in a horizontal or vertical position or for drilling long or deep holes, a feed mechanism is provided for mechanically forcing the drill bit against the material being drilled. The drill steels or rods used in these operations may be up to 10 feet or more in length and under the pressure of the feed mechanism or if the drill becomes misaligned with the hole being drilled tend to bend and bow. A second centralizer, a midpoint centralizer, is provided to attempt to prevent deflection of the drill steel. These midpoint centralizers generally take the form of heavy, drill steel encircling members attached to the drill feed mechanism or some other rigid base which forceably restrain the drill rod against deflection. The position of the centralizer is controlled so that it also moves foward as the drill is advanced but at about one-half the speed at which the drill is advanced so that the centralizer constantly maintains a position approximately at the midpoint of the exposed portion of the drill steel between the point where the drill bit and drill steel enter the material being drilled and the drill mechanism. Such prior midpoint centralizers cause excessive noise and friction and since they fit tightly around the drill steel or have minimal clearance, they reduce the length to which the operator can drill with a given length of drill steel by the width of the coupling by which the drill steel is attached to the striking bar of the drill since the coupling will not pass through the centralizer.